Descriptions

The physiological response of juvenile steelhead (Oncorhynchus mykiss) to prolonged heat stress
was examined by exposing replicated groups of fish to 25 consecutive days at 15°C, 23°C, and
25°C followed by a 55 day recovery period at 15°C. We found that at temperatures ≥25°C,
steelhead consumed significantly less food per day and had elevated feed conversion rates but
experienced slower growth, reduced body size, lower body fat, and elevated heat shock protein
70 (hsp 70) levels relative to fish 2°C and 10°C cooler. Growth decreased 24.4% and 27.1% for
length and mass, respectively, between 15°C and 23°C, and an additional 60% and 56.5%
between 23°C and 25°C during exposures. While growth increments and lipid levels recovered
to control levels after water temperature was reduced, body size of the 25°C exposed fish lagged
throughout the experiment. Our results indicate a temperature threshold after which steelhead
exposed to semi-chronic thermal stress incur a physiological debt. Heat shock protein 70 levels
were detectable up to 25 days post-stress in fin and liver tissues, providing evidence that this is a
useful metric for thermal stress that can be assessed non-lethally, an important technique relevant
for monitoring thermal-habitat restoration efforts for threatened and endangered salmonids.